This invention relates to a scrubber apparatus that employs equipment for generating streams of bubbles to mix with a liquid or liquid-like medium by which means undesirable elements can be removed or changed to a more benign form. In a specific, non-limiting example of an application for which the invention is suited, the scrubber can be used to remove particulate from the exhaust gases of an internal combustion engine.
In light of environmental concerns, in recent times there has been a greater emphasis on the reduction of pollutants emitted in smoke plumes, whether of factories, electricity generating stations, vehicles or ships. Similarly there has also been an emphasis on the removal, or conversion, of toxic chemicals emitted from industrial processes, whether in the pulp and paper, plastics, or other industries. There has also been a desire to reduce the heat emitted by engine exhaust systems, whether for the purpose of achieving greater economies by trapping and re-using waste heat for secondary and tertiary activities or for reducing the infra-red heat signature of an engine intended for military use. Further, a scrubber may, as one of its features, not only remove undesired elements, but may also reduce the noise of an exhaust flow.
There are many examples of specific instances when scrubbing is desirable. For example it may be desired to remove gaseous and fine particulate matter contaminants, odorous compounds and other undesirable elements from exhaust gases emanating from combustion of fossil fuels, whether gas, fuel oil, diesel oil and other petroleum products. The fuels are commonly used in marine diesel engines and boilers, diesel engines used for transportation and construction equipment, whether for highway vehicle use, forestry equipment, mining, or other purposes. In some instances use of a water scrubbing medium is also desired to discourage or eliminate spark emission.
In another field, it is desirable to scrub exhaust gases emanating from industrial processes such as chemical processes, heat transfer processes, food preparation, agricultural operations, mechanical parts cleaning, paint spray operations and similar processes. Similarly, it may be desired to treat products of the combustion of solid, liquid and gaseous fuels such as biomass, coal, coal water slurry, coal and limestone water slurry, coal methanol slurry. Further still, scrubbing may be required for products of combustion from incineration systems for the thermal destruction of solid, liquid or gaseous waste products. These can include industrial and municipal wastes, biomedical wastes, hazardous and pathological solid and liquid wastes, and solids and liquids contaminated with toxic, hazardous, and pathological wastes, accidental hazardous and dangerous waste spills, and similar waste products.
In another application, it may be desired to inject air and other gases into liquid chemical or liquid biomass, or liquid chemical and liquid biomass solutions. Examples of such solutions include liquors from industrial processes such as pulp and paper processes, municipal sewage, agricultural operations, food preparation liquid waste, and similar liquid systems. There are, of course, many other examples of situations in which scrubbing technology generally, and the principles of the present invention in particular, can be applied.
Scrubbers of various types are known. Removal of fine particles of dust, oxides of sulfur, odorous compounds, and similar contaminants from gas streams is a priority for environmental control abatement programs developed by regulatory agencies to minimize the impact of industrial processes on the natural environment. Devices currently in use for removal of pollutants include cyclones, bag filters, electrostatic precipitators, and high energy scrubbers. Typically the input to output efficiency of these devices range from 85% to xe2x89xa799.99%, with the high energy scrubbers being the most efficient, and the cyclone and inertial separators the least. Input to output efficiency is defined as the total concentration of particles of all size ranges in the outlet gas stream from the system as a percentage of the concentration in the total input to the gas cleaning unit.
The type of unit for a specific application is determined by a number of factors including type of industrial process, type and size of particle released, temperature of the gas stream, process economics, land use adjacent to the site, and a number of other factors. High energy scrubbers using limestone and water slurry scrubbing solutions have been successfully used to scrub sulphur from the combustion gases produced when burning sulfur containing fuels, such as coal, heavy fuel oil, and so on.
A common method of scrubbing, for example, exhaust gases, is to spray a scrubbing medium, such as water, across the exhaust gas passage, or to force the exhaust gases through a continuously fed curtain of water, or along a channel with wetted sides. These technologies for scrubbing fine particles from gaseous streams have relied on mechanical shear systems to produce large quantities of fine droplets of scrubbing solution. In each instance droplet surface area is the controlling parameter determining the efficiency of the scrubber. To increase scrubber droplet surface area for a given water mass, the average droplet diameter must decrease. The energy required to decrease the average droplet size and thus increase the average droplet surface area increases sharply. Thus the efficiency of conventional scrubbers for fine particle removal is a function of the energy input as measured by the pressure loss across the scrubber. Typical high efficiency scrubbers ( greater than 99% efficiency) operate with pressure drops in the range of 45-60 inches of water. Such units have high capital costs, and high energy and maintenance costs.
As the ratio of fine (xe2x89xa674 micron) particles to coarse (xe2x89xa775 microns) increases in the gas stream the degree of difficulty of achieving high collection efficiency increases. Similarly, chemical reactions with gaseous products and/or contaminants is a surface controlled phenomenon.
The conceptual opposite of this conventional approach is to force jets or streams of gas into baths of liquid, the gases being forced into the liquid at some depth below the free surface of the liquid. U.S. Pat. No. 4,300,924 of Coyle, issued Nov. 17, 1981 describes a device for scrubbing diesel engine exhausts by driving the exhaust gases through a straight pipe into a tank of water, and allowing the exhaust gases to bubble through the water. The Coyle apparatus operates when the head of the exhaust gases is sufficient to force them out the plain cut end of the pipe. There is no indication that Coyle considered whether bubble size increases as the flow of exhaust gases increases.
Swiss Patent 629 972 of Lxc3xcthi et al, issued May 28, 1992, shows a scrubber having one round cylinder nested within another. Gases enter the annular space between the cylinders through a targeted inlet. The bottom of the scrubber is filled with a scrubbing fluid. An array of paddles is located to generate a swirling effect as the gases pass through the liquid to reach the inside of the inner cylinder. Although at least one embodiment permits variable pitch paddles, the paddles are relatively for apart so that the flow passages are wide. The device also lacks a straightening or vortex breaker section to encourage bottom settling.
The mechanism of the scrubbing process appears to be a complex one involving two phase flow. It appears that the process is analogous to a heat transfer or mass transfer phenomenon, or both at the same time, in which the efficiency can be related to one or more of the applicable, Reynolds, Prandtl, Schmitt, Sherwood and Nusselt numbers. For heat and mass transfer, generally, it is advantageous to decrease the transport distance, and increase the cross section of the transport path. As concerns path length, since the Prandtl number for a liquid scrubbing medium, such as water, is typically an order of magnitude greater than Prandtl numbers for gases, it appears that the critical heat and mass transfer distance is related to the characteristic dimension of the bubbles, for which the mean bubble diameter is a proxy. Similarly, the cross section of the interface between the gas and liquid phases of the mix is defined by the surface area of the bubbles, a number that is, again, related to mean bubble diameter. Empirically, it is the observation of the present inventors that the efficiency of scrubbing increases as mean bubble size decreases per unit of exhaust gals flow.
Relating the scrubbing phenomenon, by analogy, to the heat and mass transfer phenomena, as the mean bubble diameter decreases the interacting surface area interface between the gas and liquid phases increases per unit volume of either gas or liquid. This decreases the mean transport distance within the low density, low thermal conductivity gas phase, as bubble size decreases. Jets, or streams, of bubbles released in the liquid in a manner to increase the turbulence of the mix still further enhance scrubbing efficiency. That is, a jet of relatively small, relatively high velocity bubbles with tend to result in scrubbing that is more, effective than a flow of relatively large, low velocity bubbles for the same flowrate. Inasmuch as both heat and mass transfer phenomena are time dependent, it is also advantageous to encourage retention of small sized bubbles for a relatively lengthy period of time.
In summary, it would be advantageous to increase the gas retention time within the liquid scrubbing solution, to increase the level of turbulence and mixing within the scrubbing solution, to reduce the bubble size, and thereby to increase the reaction surface area per unit of flow, to improve the circulation of the scrubbing liquid, or liquid like, medium.
In one aspect of the invention there is a member for a scrubber comprising a conduit having defined therein an intake for receiving gases to be scrubbed. The conduit has a wall. Porting is defined in the wall, the porting being for immersion in a scrubbing medium. When so immersed, the porting extends from a first depth to a second depth. The conduit has a passage for transporting the gases from the intake to the porting. The porting includes at least one flow splitter for encouraging formation of more than one stream of bubbles through the porting.
In an additional feature of that aspect of the invention, the porting is shaped to encourage turbulent mixing of the gases with the scrubbing medium. In a further additional feature of that additional feature, the porting is angled whereby gases exiting said porting impart angular momentum to the scrubbing medium in the bath. In yet a further additional feature, the conduit is a cylindrical pipe having a longitudinal axis. The pipe has a pipe wall. The porting is an array of slots let through the pipe wall at an angle to release the gases into the scrubbing medium in a direction having a component normal to said pipe wall and a component tangential to said pipe wall and perpendicular to said longitudinal axis.
In another additional feature of that aspect of the invention, the porting is arrayed to present a greater flow area as the head of the gases increases. In another additional feature of that aspect of the invention, the conduit has an effective cross sectional flow area and the effective cross sectional flow area of the porting is less than the effective cross sectional flow area of the conduit.
In another aspect of the invention, there is a scrubber for scrubbing a gas. It comprises a scrubbing vessel for containing a scrubbing medium. The scrubbing vessel has a reaction zone and a quiescent zone. A conduit has defined therein an intake for receiving gases to be scrubbed, outlet porting, and a passage for transporting the gases from said intake to said porting. The conduit is mounted to present the porting in an immersed position relative to the scrubbing medium in the reaction zone. The porting includes at least one turbulence generator for encouraging turbulent mixing of gases exiting the conduit with the scrubbing medium.
In an additional feature of that aspect of the invention, reaction zone and the quiescent zone are separated by a turbidity interrupter. In another additional feature of that additional feature, the turbidity interrupter is chosen from the set of turbidity interrupters consisting of at least one of (a) a curtain wall partition; and (b) a vorticity breaker. In another additional feature of that aspect of the invention the porting includes an array of separated fingers having gas flow apertures defined therebetween. In still another feature of that aspect of the invention, the conduit has a peripheral wall extending between a first depth and a second depth. The turbulence generators are elements of the porting let through said peripheral wall.
In still another additional feature of that aspect of the invention, conduit has a peripheral wall that extends in a longitudinal direction. The direction has a vertical component relative to the scrubbing medium. The turbulence generators are elements of the porting let through the peripheral wall in a direction having a component normal to the wall and another component horizontally tangential to the wall. In an additional feature of that additional feature, the turbulence generator is angled at an angle in the range of 10xc2x0 to 75xc2x0 relative to said normal wall.
In still yet another additional feature of that aspect of the invention the scrubber further comprises a scrubbing fluid supply system mounted to introduce a flow of scrubbing medium into the conduit. In still another additional feature of that aspect of the invention, the member is an intake member mounted amidst a scrubbing fluid reservoir. The conduit has an inner wall, and the intake has a weir mounted to encourage scrubbing fluid from the reservoir to flow along the wall. In yet another additional feature of that aspect of the invention, the scrubber has a secondary scrubber stage mounted to intercept gases emanating front the porting. The secondary scrubber stage also has turbulence generators mounted to lie immersed in the scrubbing medium. In an additional feature of that additional feature, the secondary scrubber stage has a trap for the gases. A turbulence generator of the secondary scrubber stage is let through the trap at an angle to impart a component of momentum to gases exiting therefrom that is opposed to the horizontally tangential component of the turbulence generator of the conduit.
In yet a further additional feature of that aspect of the invention, the conduit is a cylindrical pipe having a pipe wall and a longitudinal axis. The porting is an array of apertures let through the pipe wall. The porting extends between a first depth and a second depth relative to said reaction zone. At least one of the apertures is let through the wall at an angle having a direction that has a component normal to the pipe wall and a component tangential to the pipe wall and perpendicular to the longitudinal axis. The pipe has a barrier planed about the periphery thereof to intercept bubbles emanating from the apertures. The barrier has a second set of apertures let at an angle therethrough in a submerged location relative to the reaction zone, for encouraging the formation of bubbles. The scrubber has at least one turbidity breaker between the reaction zone and the quiescent zone, to permit exchange of scrubbing fluid therebetween, and the barrier is surrounded by a settling column.
In another aspect of the invention, there is a method for passing a gas through a liquid. The method comprises the steps of forcing the gas through porting submerged in the liquid to form bubbles, encouraging the breaking of the bubbles, and settling the liquid in a quiescent zone to permit bubbles entrained in the liquid to separate out.
In an additional feature of that aspect of the invention, the step of forcing includes the step of directing the gas into the liquid at an angle for imparting momentum thereto. In another additional feature of that method, the step of forcing includes compelling the gas to move from a gas manifold through the porting to a mixing zone. In an additional feature of that additional feature, the step of settling includes permitting the liquid to settle in a settling column physically segregated from the mixing zone. In yet a further additional feature of that additional feature, the step of settling includes passing the liquid through a vorticity breaker.